Abstract : This report gives results of computations for the exposure field from point isotropic sources of Cobalt-60 and Cesium-137 near the surface of a semi-infinite concrete slab. A modification of the Chilton-Huddleston differential albedo formula developed for the vacuum/concrete interface situation is used. To modify the results to include the air effect, computation was made to account for the single-scattered contribution. The higher order air scattering contribution, which is smaller, was estimated approximately from infinite buildup factors in air. The final results, with horizontal source-detector distances out to 50 feet, are presented in comparison with various experimental results. The computational results are estimated to have an uncertainty of about 1%. Agreement with previous work is good in some cases, but under some conditions appreciable discrepancies exist. The preponderance of evidence supports the theoretical results and they are recommended for future use. (Author)